In many optical systems, it is necessary to manipulate a propagating optical signal from one plane to another within a substrate-based optical arrangement. Electro-optic devices, such as waveguides, generally feature two primary planes—one parallel to the substrate and one perpendicular (normal) to that substrate. Often, the orientation of the optical beam needs to be turned 90° to redirect it from one plane to another.
FIGS. 1-3 illustrate a prior art arrangement that is used to provide re-direction of an optical signal from one orientation to another. FIG. 1 is a side view, illustrating an optical waveguide 1 formed along a portion of an optical substrate 2. An optical beam O exiting endface 3 of waveguide 1 will expand in all three dimensions as it propagates outward from waveguide 1, forming a conic wavefront as shown in the isometric view of FIG. 2. The Cartesian XYZ coordinates as will be used throughout this discussion are illustrated in both FIGS. 1 and 2, where the Z-axis is defined as the optical axis of waveguide 1 and the XY plane defines endface 3.
In order to turn optical beam O and re-direct it into a waveguide in another plane (for example, “above” waveguide 1), an angled reflecting surface is often used, shown as reflecting surface 4 in FIG. 1. Reflecting surface 4 is disposed along the output signal path from waveguide 1, in this case defined as the Z-axis of the system. Reflecting surface 4 will intercept the propagating beam and, in this configuration, direct it upwards. Generally referred to in the optics art as a “turning mirror”, reflecting surface 4 may advantageously be formed of the same silicon material as the remainder of the arrangement and fabricated using CMOS processing to create the desired angle of reflection. For the purposes of the present discussion, the phrase “turning mirror” will be hereinafter used to describe this component.
Inasmuch as waveguide 1 terminates as a perpendicular facet at endface 3 (see FIG. 3 for an enlarged illustration of endface 3), expansion of the beam in the XY plane will continue, causing the signal power to spread across a relatively large surface area, as illustrated by the conic wavefront shown in FIG. 2. Moreover, the beam expansion will continue after the signal has been re-directed by turning mirror 4. Plane 5 and associated spot size image A in FIG. 1 illustrates the degree of expansion which has occurred by the time optical beam O has exited waveguide 1 and been re-directed by turning mirror 4.
This constant expansion thus results in reducing the optical power present at any point along a surface (such as plane 5). When re-directing an optical signal into a waveguide, optical receiving device, or the like, it would be preferable to control the spot size of the re-directed beam so as to improve the coupling efficiency of the propagating optical signal into the other waveguide, optical receiving device, or the like. That is, it is desirable to limit the expansion of the optical wavefront along axes perpendicular to the direction of signal propagation (i.e., when propagating along the Z-axis, limit expansion in the XY plane).